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<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        var arr=[1,2,3,4,5]

        Array.prototype.myReduce=function(callback){
            var t=this[0];
            for(var i=1;i<this.length;i++){
                t=callback(t,this[i],i,this)
            }
            return t;
        }

        var res=arr.myReduce(function(prev,item){
            return prev+=item;
        })
        console.log(res);
    </script>
</body>
</html>